Claude review: Add "link bpc" DRM property

[Claude Code Review Bot <claude-review@example.com>]

Overall Series Review

Subject: Add "link bpc" DRM property
Author: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Patches: 3
Reviewed: 2026-03-12T06:57:50.360536

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This is a 2-patch series adding a new immutable "link bpc" DRM connector property that exposes the actual bits-per-component achieved on a display link, letting userspace detect when bit depth has been degraded (e.g. due to bandwidth constraints). Patch 1 adds the core DRM infrastructure and HDMI helper integration; patch 2 adds amdgpu support.

The design is clean and the approach is reasonable. There are a few issues worth discussing:

**Timing concern in `commit_tail`**: The link_bpc property update and uevent fire *before* `funcs->atomic_commit_tail()` runs. This means userspace is notified of a bpc change before the hardware has actually committed the new state. For amdgpu specifically (which has its own `atomic_commit_tail`), this means the uevent fires before the display hardware is programmed. Ideally this should happen *after* the hw commit completes.

**Clamping lower bound of 8**: The property range and clamping both use 8 as the minimum, but some displays legitimately run at 6 bpc (e.g. certain eDP panels). If 6 bpc is "experimental internal" for amdgpu, that's fine for patch 2's local helper, but hard-coding 8 as the universal lower bound in the core DRM property (patch 1) seems potentially limiting.

**Missing link_bpc reset on disconnect**: When a connector is disabled/disconnected, `link_bpc` is never explicitly reset. It will carry the stale value from the last active state via `memcpy` in `duplicate_state`. This means the property could report a stale bpc value on a disconnected connector.

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